METHOD FOR THE OXIDATION OF VICINAL DIOLS, INCLUDING CARBOHYDRATES

Abstract: A method is described for the oxidation of vicinal diols, including carbohydrates, to dicarboxy compounds by reaction of the diol in an aqueous medium with hydrogen peroxide or via an electrochemical route, in the presence of a catalytic amount of a transition metal salt. Oxidation with hydrogen peroxide in the presence of a catalytic amount of an alkali metal halide, alkaline earth metal halide or ammonium halide is also described.

Description: Method for the oxidation of vicinal diols. including carbohydrates The invention relates to a method for the oxidation of diols, including carbohydrates, which possess two neighbouring secondary alcohol groups (vicinal diols). The oxidation of such diols leads to cleavage of the C-C bond and results in products like dialdehydes and eventually to dicarboxylic acids. When the diol is acyclic, this results in degradation of the molecule; for example complete oxidation of one molecule of 2,3- butanediol yields two molecules of acetic acid. When the diol is cyclic, such as cyclohexanediol or a carbohydrate, oxidation leads to ring-opened products: in the case of cyclohexanediol, the end product is adipic acid, in the case of carbohydrates, the end product is a dicarboxy- carbohydrate, e.g. a chain of dicarboxy acetals. Dicarboxy compounds of this type can be used as complex-forming agents for metal ions, such as calcium and magnesium, and can therefore, for example, be used in detergents as replacements for phosphates. Several variants have been described for the oxidation of vicinal diols including carbohydrates. Known oxidising agents for this purpose are periodic acid, lead(IV) salts, chlorite and hypochlorite. It is also known to oxidise carbohydrates using hydrogen peroxide in the presence of sodium tungstate as catalyst (M. Floor et al, Starch, 4l, 303~309 (1989)); with this method oxidation products other than dicarboxy-carbo- hydrates are also formed and, moreover, chain degradation takes place. The oxidation of polysaccharides using a catalytic amount of bromide with the formation of dicarboxy-polysaccharides is described in EP 427,349 and WO 91.17189. In this method the bromide is converted to hypobromite by hypochlorite or via an electrochemical route. The known methods for the oxidation of vicinal diols have the disadvantage that they lead to at least equivalent amounts of salt in the end product. The salts present separation problems, restrict the possible applications of the product, and/or lead to environmental problems. Moreover, the yield of dicarboxy product is sometimes inadequate, or long reaction times are needed. A method for the oxidation of vicinal diols has now been found with which no salt or only small amounts of salt are formed and which leads to a yield of higher than 90 % of the desired dicarboxy products within short reaction times. In accordance with the method according to the invention, the diol is oxidised in an aqueous medium with hydrogen peroxide or via an electrochemical route in the presence of a catalytic amount of a metal salt which is a metal halide or a salt of a transition metal. An aqueous medium is understood herein as a medium that consists of at least 50% water. If necessary in view of the solubility of the substrate to be oxidised, an organic solvent such as a ketone, ether, ester, amide or sulphoxide may be present. However, it is preferred to carry out the oxidation in the substantial absence, i.e. less than 10 , in particular less than _\% , of organic solvents. The diols which can be oxidised by the method according to the invention include all vicinal diols, both acyclic and cyclic, both ali¬ phatic and aromatic-aliphatic, both monomeric and polymeric. Examples are 2,3-butanediol, cyclohexanediol, higher cycloalkane- and cycloalkene- diols, 1,2-diphenylethanediol, and the like. The method is particularly useful for oxidising carbohydrates, i.e. compounds which, in addition to at least hydroxy groups, also possess a ketone or aldehyde function, which may or may not be cyclised with a hydroxy group into an acetal. The carbohydrates may be mono-, oligo- and polysaccharides and derivatives thereof which contain a vicinal diol group, such as ribose, glucose, galactose, fructose and the oligo- and polysaccharides based thereon. Important examples of usable carbohydrates are starch (poly-α-anhydro- glucose) , cellulose (poly-α,β-anhydroglucose) and inulin (mainly poly- anhydrofructose) . Derivatives of carbohydrates such as sugar alcohols, sugar acids, amines, esters, and amides, for example sorbitol, mannitol, polyglucuronic acid, acetamido glucose, are also successfully oxidised using the method of the invention. The method according to the invention can lead to high yields of - dicarboxy products, as a rule higher than 90 % of the theoretical yield. However, the method is also suitable for producing partly oxidised products. With this method the oxidation leads to hardly any chain degradation of polymeric substrates. In addition, the crude product contains only small amounts of salts, specifically the amounts which correspond to the catalytic amount of metal salt used. The term dicarboxy compound is used to denote the original compound in which the vicinal diol group has been converted to two carboxyl groups with breaking of the C-C bond, and thus possibly with ring opening. Similarly the term dicarboxy-carbohydrate is used. According to a preferred variant of the method according to the invention, the diol is oxidised using an approximately equivalent amount of hydrogen peroxide in the presence of a catalytic amount of a halide salt. The halide ion, which in this case can be chloride, bromide or iodide, is used as an intermediate in the oxidation, i.e. it is oxidised by hydrogen peroxide to higher oxidation states, in particular hypo- halite, and it is reduced by the diol to be oxidised. The counterion of the halide ion can advantageously be an alkali metal or alkaline earth metal ion, such as in sodium chloride or magnesium bromide. Ammonium halides, especially quaternary ammonium salts such as tetramethyl, tetra- ethyl ammonium halides and the like, can also be used. This method is very attractive since it produces only low amounts of unwanted by¬ products: the hydrogen peroxide used yields water, and the halide salt is recycled and, besides, is a relative harmless salt such as sodium chloride. An equivalent amount of hydrogen peroxide is an amount of 3 mol of hydrogen peroxide per mol unit of diol, in accordance with the empirical equation: -CH0H-CH0H- + 3 H 2 0 2 → 2 -C00H + 4 H 2 0. In this context a catalytic amount of metal salt or halide salt is understood to be an amount which is appreciably less than an equimolar amount with respect to the monosaccharide units, that is to say less than 50 % of the equimolar amount and in particular less than 25 % thereof. Preferably 0.02 - 0.20 mol, and more preferentially 0.05-0.15 mol, of halide salt is used per mol of diol unit. With this variant of the method, virtually quantitative conversion to dicarboxy product is achieved within a few hours to a few days. The reaction temperature can vary from 0 °C to 100 °C, in particular from 20 to 80 °C, more in particular from 4θ-8θ°C. It has been found that the pH can be varied over a wide range from about 4 to high pH; a pH of between 5 and 9 is preferably employed. Best results are obtained if the pH is around neutrality, i.e. between 6 and 8. According to another variant of the method according to the invention, the metal salt is a salt of a transition metal, in which case the oxidation can be carried out using hydrogen peroxide or via an electrochemical route. The transition metal can be any metal that can exist in various oxidation states, such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Ag from the fourth row of the Periodic Table of the Elements and the corresponding metals from higher rows of the Periodic Table. Salts of transition metals of group IB, i.e. copper, silver and gold, especially copper, are preferred. In particular halides, and more especially chlorides, bromides or iodides, of the transition metals are used. In this case a catalytic amount of metal salt can be an amount of, for example, 1 - 25 % , in particular 2 - 10 % , of an equimolar amount of vicinal diol. With this variant of the method virtually quantitative yields of dicarboxy products are already obtained within a few hours. The reaction temperature and pH can be the same as indicated above, in particular 40- 80°C and pH 6-8, respectively. For electrochemical oxidation the usual conditions can be main¬ tained and, for example, platinum electrodes are used. In the electro¬ chemical oxidation the halide ion or transition metal is continuously brought from a lower to a higher oxidation state, it being assumed that the metal in the higher oxidation state or the hypohalite respectively, oxidises the vicinal diol. As explained above, the method can be used to produce dicarboxy products in high yield, but it can also be used to produce partially oxidised products, i.e. products which, in addition to carboxy groups, also contain alcohol groups and/or aldehyde groups. Preferably, the degree of oxidation is at least 10 % , more preferably at least 50 % of the theoretical maximum degree, i.e. where all diol groups have been transformed into dicarboxy groups. Example 1 10 g of inulin, isolated from chicory, having a DP of about 8 are dis¬ solved in 300 ml of water. 300 mg of sodium chloride are added to this solution. The system is heated to 50 °C. 18 g of hydrogen peroxide (35 % w/v) are added to this mixture over a period of one hour. The reaction mixture is kept at pH 5- After 24 hours, the reaction mixture is evaporated. After drying, the product is obtained in a yield of higher than 90 % . After dissolving in D 2 0, this product is analysed with the aid of ** H-NMR and 1 C-NMR. The spectra show that dicarboxyinulin has been obtained. Example 2 10 g of inulin, isolated from dahlias, having a DP of about 30 are dissolved in 300 ml of water. The same procedure as described in Example 1 is followed. The yield of dicarboxyinulin determined after drying is higher than 90 % . Characterisation of the product was carried out in the same way as described in Example 1. Example 3 10 g of inulin, isolated from Jerusalem artichokes, having a DP of about 8 are dissolved in 300 ml of water. The same procedure as described in Example 1 is followed. The yield of dicarboxyinulin determined after drying is higher than 90 % . Characterisation of the product was carried out in the same way as described in Example 1. Example 4 10 g of inulin, isolated from chicory, having a DP of about 8 were converted to dicarboxyinulin as described in Example 1, except that the reaction was carried out at pH 7- Yield higher than 0 % ; characterisation of the product using ** H-NMR and 13 C-NMR. Example 5 10 g of inulin, isolated from dahlias, having a DP of about 30 were converted to dicarboxyinulin as described in Example 1, except that the reaction was carried out at pH . Yield higher than 90 %; characterisation of the product using * Η-NMR and 13 C-NMR. Example 6 10 g of inulin, isolated from Jerusalem artichokes, having a DP of about 8 were converted to dicarboxyinulin as described in Example 1, except that the reaction was carried out at pH . Yield higher than 90 % ; characterisation of the product using --H-NMR and 13 C-NMR. Example 7 10 g of inulin, isolated from chicory, having a DP of about 8 were converted to dicarboxyinulin as described in Example 1, except that the reaction was carried out at pH 9- Yield higher than 90 % ; characterisation of the product using ** H-NMR and 13 C-NMR. Example 8 10 g of inulin, isolated from dahlias, having a DP of about 30 were converted to dicarboxyinulin as described in Example 1, except that the reaction was carried out at pH _) . Yield higher than 90 % ; characterisation of the product using ** H-NMR and 13 C-NMR. Example 9 10 g of inulin, isolated from Jerusalem artichokes, having a DP of about 8 were converted to dicarboxyinulin as described in Example 1, except that the reaction was carried out at pH 9- Yield higher than 90 % ; characterisation of the product using ** H-NMR and 13 C-NMR. Example 10 10 g of inulin, isolated from chicory, having a DP of about 8 are dissolved in 300 ml of water. 0.2 mmol of copper(II) chloride is added to this solution. The pH is then adjusted to 5 and the temperature is raised to 60 °C. 18 g of hydrogen peroxide (35 % /v) are added to this reaction mixture over a period of 15 minutes. After two hours, the reaction mixture is evaporated. After drying, the yield is higher than 90 % . The product is characterised as dicarboxyinulin with the aid of ** H- NMR and 13 C-NMR. Furthermore, ** H-NMR and 13 C-NMR of a sample taken from the reaction mixture show that virtually no formic acid (or the salt thereof) or carbonate salts are formed as reaction by-products. Example 11 10 g of inulin, isolated from dahlias, having a D ~ P of about 30 are dissolved in 300 ml of water. The same procedure as described in Example 10 is followed. The yield of dicarboxyinulin determined after drying is higher than 90 % . Characterisation of the product and the reaction mixture were carried out in the same way as described in Example 10. Example 12 10 g of inulin, isolated from Jerusalem artichokes, having a DP of about 8 are dissolved in 300 ml of water. The same procedure as described in Example 10 is followed. The yield of dicarboxyinulin determined after drying is higher than 90 % . Characterisation of the product and the reaction mixture were carried out in the same way as described in Example 10. Example 13 10 g of inulin, isolated from chicory, having a DP of about 8 were converted to dicarboxyinulin as described in Example 10, except that the reaction was carried out at pH 1. Yield higher than 90 % ; characterisation of the product using ** H-NMR and 13 C-NMR. Example 14 10 g of inulin, isolated from dahlias, having a DP of about 30 were converted to dicarboxyinulin as described in Example 10, except that the reaction was carried out at pH " . Yield higher than 90 % ; characterisation of the product using --H-NMR and 13 C-NMR. Example 15 10 g of inulin, isolated from Jerusalem artichokes, having a DP of about 8 were converted to dicarboxyinulin as described in Example 10, except that the reaction was carried out at pH 1. Yield higher than 90 % ; characterisation of the product using ** H-NMR and 13 C-NMR. Example 16 10 g of inulin, isolated from chicory, having a DP of about 8 are dissolved in 300 ml of water. 0.2 mmol of copper(II) chloride is added to this solution. The pH is then adjusted to 5 and the temperature is raised to 60 °C. By means of electrochemical oxidation, the copper(II) chloride is re-oxidised continuously. After two hours, the reaction mixture is evaporated. After drying, the yield is higher than 90 % . The product is characterised as dicarboxyinulin with the aid of ** H-NMR and 13 C-NMR. Furthermore, ** H-NMR and 13 C-NMR of a sample taken from the reaction mixture show that no formic acid (or the salt thereof) or carbonate salts are formed as reaction by-products. Example 17 10 g of inulin, isolated from dahlias, having a DP of about 30 are dissolved in 300 ml of water. The same procedure as described in Example 16 is followed. The yield of dicarboxyinulin determined after drying is higher than 0 % . Characterisation of the product and the reaction mixture were carried out in the same way as described in Example 16. Example 18 10 g of inulin, isolated from Jerusalem artichokes, having a ~ DP of about 8 are dissolved in 3 0 ml of water. The same procedure as described in Example 16 is followed. The yield of dicarboxyinulin determined after drying is higher than 90 % . Characterisation of the product and the reaction mixture were carried out in the same way as described in Example 16. Example 19 10 g of inulin, isolated from chicory, having a DP of about 8 were converted to, and characterised as, dicarboxyinulin as described in Example 16, except that the reaction was carried out at pH 1. Yield higher than 90 % characterisation of the product using ** H-NMR and 13 C- NMR. Example 20 10 g of inulin, isolated from dahlias, having a DP of about 30 were converted to, and characterised as, dicarboxyinulin as described in Example 16, except that the reaction was carried out at pH 7• Yield higher than 90 % ', characterisation of the product using ** H-NMR and 13 C- NMR. Example 21 10 g of inulin, isolated from Jerusalem artichokes, having a ~ DP of about 8 were converted to, and characterised as, dicarboxyinulin as described in Example 16, except that the reaction was carried out at pH 7- Yield higher than 90 % characterisation of the product using ** H-NMR and 1 C- NMR.

Claims: Claims 1. Method for oxidising vicinal diols by reaction of the diol in an aqueous medium with hydrogen peroxide or via an electrochemical route, in the presence of a catalytic amount of a metal salt which is a halide salt or a salt of a transition metal. 2. Method according to Claim 1, wherein the metal salt is an alkali metal halide, alkaline earth metal halide or ammonium halide and the oxidation is carried out with hydrogen peroxide. 3. Method according to Claim 1, wherein the metal salt is a halide of a transition metal. 4. Method according to Claim 1 or 3. wherein the transition metal is a metal from group IB. 5. Method according to one of Claims 1-4, wherein a pH of between 5 and 9 is employed. 6. Method according to Claim 5. wherein a pH between 6 and 8 is used. 7. Method according to one of Claims 1-6, wherein a reaction temperature of 4θ-8θ°C is used. 8. Method according to one of Claims 1-7, wherein the aqeuous medium is substantially free of organic solvents. 9. Method according to one of Claims 1-8, wherein the vicinal diol is a carbohydrate, in particular a glucan or a fructan. 10. Method according to Claim 9. wherein the carbohydrate is starch, cellulose or inulin or a fraction thereof.

Inventors: DE WIT Dirk, TOURNOIS HUBERT, VEELAERT SARAH, DE WIT, DIRK, TOURNOIS, HUBERT, VEELAERT, SARAH
Applicants: INST VOOR AGROTECH ONDERZOEK, TOURNOIS HUBERT, VEELAERT SARAH, WIT DIRK DE, DE WIT, DIRK, INSTITUUT VOOR AGROTECHNOLOGISCH ONDERZOEK (ATO-DL, TOURNOIS, HUBERT, VEELAERT, SARAH
Application year: 1994
Main classification: C08B37
IPC codes: C08B15, C08B31, C08B37